SH(1)							      General Commands Manual							     SH(1)

NAME

       sh,  for, case, if, while, :, ., break, continue, cd, eval, exec, exit, export, login, read, readonly, set, shift, times, trap, umask, wait
       - command language

SYNOPSIS

       sh [ -ceiknrstuvx ] [ arg ] ...

DESCRIPTION

       Sh is a command programming language that executes commands read from a terminal or a file.  See invocation for the meaning of arguments to
       the shell.

       Commands.
       A  simple-command is a sequence of non blank words separated by blanks (a blank is a tab or a space).  The first word specifies the name of
       the command to be executed.  Except as specified below the remaining words are passed as arguments to the  invoked  command.   The  command
       name  is passed as argument 0 (see execve(2)).  The value of a simple-command is its exit status if it terminates normally or 200+status if
       it terminates abnormally (see sigvec(2) for a list of status values).

       A pipeline is a sequence of one or more commands separated by |.  The standard output of each command  but  the	last  is  connected  by  a
       pipe(2) to the standard input of the next command.  Each command is run as a separate process; the shell waits for the last command to ter-
       minate.

       A list is a sequence of one or more pipelines separated by ;, &, && or || and optionally terminated by ; or &.  ; and & have  equal  prece-
       dence  which  is lower than that of && and ||, && and || also have equal precedence.  A semicolon causes sequential execution; an ampersand
       causes the preceding pipeline to be executed without waiting for it to finish.  The symbol && (||) causes the list following to be executed
       only if the preceding pipeline returns a zero (non zero) value.	Newlines may appear in a list, instead of semicolons, to delimit commands.

       A  command is either a simple-command or one of the following.  The value returned by a command is that of the last simple-command executed
       in the command.

       for name [in word ...] do list done
	      Each time a for command is executed name is set to the next word in the for word list.  If in word  ...	is  omitted,  in  "$@"	is
	      assumed.	Execution ends when there are no more words in the list.

       case word in [pattern [ | pattern ] ... ) list ;;] ... esac
	      A  case command executes the list associated with the first pattern that matches word.  The form of the patterns is the same as that
	      used for file name generation.

       if list then list [elif list then list] ... [else list] fi
	      The list following if is executed and if it returns zero the list following then is executed.  Otherwise, the list following elif is
	      executed and if its value is zero the list following then is executed.  Failing that the else list is executed.

       while list [do list] done
	      A  while	command  repeatedly  executes the while list and if its value is zero executes the do list; otherwise the loop terminates.
	      The value returned by a while command is that of the last executed command in the do list.  until may be used in place of  while	to
	      negate the loop termination test.

       ( list )
	      Execute list in a subshell.

       { list }
	      list is simply executed.

       The following words are only recognized as the first word of a command and when not quoted.

	      if then else elif fi case in esac for while until do done { }

       Command substitution.
       The  standard  output  from  a  command	enclosed in a pair of back quotes (``) may be used as part or all of a word; trailing newlines are
       removed.

       Parameter substitution.
       The character $ is used to introduce substitutable parameters.  Positional parameters may be assigned values by set.  Variables may be  set
       by writing

	      name=value [ name=value ] ...

       ${parameter}
	      A  parameter  is a sequence of letters, digits or underscores (a name), a digit, or any of the characters * @ # ? - $ !.	The value,
	      if any, of the parameter is substituted.	The braces are required only when parameter is followed by a letter, digit, or	underscore
	      that  is	not to be interpreted as part of its name.  If parameter is a digit, it is a positional parameter.  If parameter is * or @
	      then all the positional parameters, starting with $1, are substituted separated by spaces.  $0 is set from argument  zero  when  the
	      shell is invoked.

       ${parameter-word}
	      If parameter is set, substitute its value; otherwise substitute word.

       ${parameter=word}
	      If  parameter is not set, set it to word; the value of the parameter is then substituted.  Positional parameters may not be assigned
	      to in this way.

       ${parameter?word}
	      If parameter is set, substitute its value; otherwise, print word and exit from the shell.  If word is omitted, a standard message is
	      printed.

       ${parameter+word}
	      If parameter is set, substitute word; otherwise substitute nothing.

       In  the	above  word  is not evaluated unless it is to be used as the substituted string.  (So that, for example, echo ${d-'pwd'} will only
       execute pwd if d is unset.)

       The following parameters are automatically set by the shell.

	      #      The number of positional parameters in decimal.
	      -      Options supplied to the shell on invocation or by set.
	      ?      The value returned by the last executed command in decimal.
	      $      The process number of this shell.
	      !      The process number of the last background command invoked.

       The following parameters are used but not set by the shell.

	      HOME   The default argument (home directory) for the cd command.
	      PATH   The search path for commands (see execution).
	      MAIL   If this variable is set to the name of a mail file, the shell informs the user of the arrival of mail in the specified file.
	      PS1    Primary prompt string, by default '$ '.
	      PS2    Secondary prompt string, by default '> '.
	      IFS    Internal field separators, normally space, tab, and newline.  IFS is ignored if sh is running as root  or	if  the  effective
		     user id differs from the real user id.

       Blank interpretation.
       After  parameter  and command substitution, any results of substitution are scanned for internal field separator characters (those found in
       $IFS) and split into distinct arguments where such characters are found.  Explicit null arguments ("" or '') are retained.   Implicit  null
       arguments (those resulting from parameters that have no values) are removed.

       File name generation.
       Following  substitution,  each  command	word  is  scanned for the characters *, ?  and [.  If one of these characters appears, the word is
       regarded as a pattern.  The word is replaced with alphabetically sorted file names that match the pattern.  If no file name is  found  that
       matches	the pattern, the word is left unchanged.  The character .  at the start of a file name or immediately following a /, and the char-
       acter /, must be matched explicitly.

       *      Matches any string, including the null string.
       ?      Matches any single character.
       [...]  Matches any one of the characters enclosed.  A pair of characters separated by - matches any character lexically between the pair.

       Quoting.
       The following characters have a special meaning to the shell and cause termination of a word unless quoted.

	    ;	&   (	)   |	<   >	newline   space   tab

       A character may be quoted by preceding it with a .  
ewline is ignored.  All characters enclosed between a  pair  of  quote  marks  (''),
       except  a  single quote, are quoted.  Inside double quotes ("") parameter and command substitution occurs and  quotes the characters  ' "
       and $.

       "$*" is equivalent to "$1 $2 ..."  whereas
       "$@" is equivalent to "$1" "$2" ... .

       Prompting.
       When used interactively, the shell prompts with the value of PS1 before reading a command.  If at any time a newline is typed  and  further
       input is needed to complete a command, the secondary prompt ($PS2) is issued.

       Input output.
       Before  a  command is executed its input and output may be redirected using a special notation interpreted by the shell.  The following may
       appear anywhere in a simple-command or may precede or follow a command and are not passed on to the invoked command.   Substitution  occurs
       before word or digit is used.

       <word  Use file word as standard input (file descriptor 0).

       >word  Use  file word as standard output (file descriptor 1).  If the file does not exist, it is created; otherwise it is truncated to zero
	      length.

       >>word Use file word as standard output.  If the file exists, output is appended (by seeking to the end); otherwise the file is created.

       <<word The shell input is read up to a line the same as word, or end of file.  The resulting document becomes the standard input.   If  any
	      character  of word is quoted, no interpretation is placed upon the characters of the document; otherwise, parameter and command sub-
	      stitution occurs, 
ewline is ignored, and  is used to quote the characters  $ ' and the first character of word.

       <&digit
	      The standard input is duplicated from file descriptor digit; see dup(2).	Similarly for the standard output using >.

       <&-    The standard input is closed.  Similarly for the standard output using >.

       If one of the above is preceded by a digit, the file descriptor created is that specified by the digit (instead of the  default	0  or  1).
       For example,

	    ... 2>&1

       creates file descriptor 2 to be a duplicate of file descriptor 1.

       If  a  command  is followed by & then the default standard input for the command is the empty file (/dev/null).	Otherwise, the environment
       for the execution of a command contains the file descriptors of the invoking shell as modified by input output specifications.

       Environment.
       The environment is a list of name-value pairs that is passed to an executed program in  the  same  way  as  a  normal  argument	list;  see
       execve(2)  and  environ(7).   The shell interacts with the environment in several ways.	On invocation, the shell scans the environment and
       creates a parameter for each name found, giving it the corresponding value.  Executed commands inherit the same environment.  If  the  user
       modifies  the  values  of  these parameters or creates new ones, none of these affects the environment unless the export command is used to
       bind the shell's parameter to the environment.  The environment seen by any executed command is thus composed of any unmodified	name-value
       pairs originally inherited by the shell, plus any modifications or additions, all of which must be noted in export commands.

       The  environment  for any simple-command may be augmented by prefixing it with one or more assignments to parameters.  Thus these two lines
       are equivalent

	      TERM=450 cmd args
	      (export TERM; TERM=450; cmd args)

       If the -k flag is set, all keyword arguments are placed in the environment, even if the occur after the command name.  The following prints
       'a=b c' and 'c':
       echo a=b c
       set -k
       echo a=b c

       Signals.
       The INTERRUPT and QUIT signals for an invoked command are ignored if the command is followed by &; otherwise signals have the values inher-
       ited by the shell from its parent.  (But see also trap.)

       Execution.
       Each time a command is executed the above substitutions are carried out.  Except for the 'special commands' listed below a new  process	is
       created and an attempt is made to execute the command via an execve(2).

       The  shell  parameter $PATH defines the search path for the directory containing the command.  Each alternative directory name is separated
       by a colon (:).	The default path is :/bin:/usr/bin.  If the command name contains a /, the search  path  is  not  used.   Otherwise,  each
       directory in the path is searched for an executable file.  If the file has execute permission but is not an a.out file, it is assumed to be
       a file containing shell commands.  A subshell (i.e., a separate process) is spawned to read it.	A parenthesized command is  also  executed
       in a subshell.

       Special commands.
       The  following commands are executed in the shell process and except where specified no input output redirection is permitted for such com-
       mands.

       #      For non-interactive shells, everything following the # is treated as a comment, i.e. the rest of the line is ignored.  For  interac-
	      tive shells, the # has no special effect.

       :      No effect; the command does nothing.
       . file Read and execute commands from file and return.  The search path $PATH is used to find the directory containing file.
       break [n]
	      Exit from the enclosing for or while loop, if any.  If n is specified, break n levels.
       continue [n]
	      Resume the next iteration of the enclosing for or while loop.  If n is specified, resume at the n-th enclosing loop.
       cd [arg]
	      Change the current directory to arg.  The shell parameter $HOME is the default arg.
       eval [arg ...]
	      The arguments are read as input to the shell and the resulting command(s) executed.
       exec [arg ...]
	      The  command  specified  by the arguments is executed in place of this shell without creating a new process.  Input output arguments
	      may appear and if no other arguments are given cause the shell input output to be modified.
       exit [n]
	      Causes a non interactive shell to exit with the exit status specified by n.  If n is omitted, the exit status is that  of  the  last
	      command executed.  (An end of file will also exit from the shell.)
       export [name ...]
	      The  given names are marked for automatic export to the environment of subsequently-executed commands.  If no arguments are given, a
	      list of exportable names is printed.
       login [arg ...]
	      Equivalent to 'exec login arg ...'.
       read name ...
	      One line is read from the standard input; successive words of the input are assigned to the variables name in order,  with  leftover
	      words to the last variable.  The return code is 0 unless the end-of-file is encountered.
       readonly [name ...]
	      The  given names are marked readonly and the values of the these names may not be changed by subsequent assignment.  If no arguments
	      are given, a list of all readonly names is printed.
       set [-eknptuvx [arg ...]]
	      -e If non interactive, exit immediately if a command fails.
	      -k All keyword arguments are placed in the environment for a command, not just those that precede the command name.
	      -n Read commands but do not execute them.
	      -t Exit after reading and executing one command.
	      -u Treat unset variables as an error when substituting.
	      -v Print shell input lines as they are read.
	      -x Print commands and their arguments as they are executed.
	      -  Turn off the -x and -v options.

	      These flags can also be used upon invocation of the shell.  The current set of flags may be found in $-.

	      Remaining arguments are positional parameters and are assigned, in order, to $1, $2, etc.  If no arguments are given, the values	of
	      all names are printed.

       shift  The positional parameters from $2...  are renamed $1...

       times  Print the accumulated user and system times for processes run from the shell.

       trap [arg] [n] ...
	      Arg  is  a command to be read and executed when the shell receives signal(s) n.  (Note that arg is scanned once when the trap is set
	      and once when the trap is taken.)  Trap commands are executed in order of signal number.	If arg is absent, all trap(s) n are  reset
	      to  their  original values.  If arg is the null string, this signal is ignored by the shell and by invoked commands.  If n is 0, the
	      command arg is executed on exit from the shell, otherwise upon receipt of signal n as numbered in sigvec(2).  Trap with no arguments
	      prints a list of commands associated with each signal number.

       umask [ nnn ]
	      The  user  file  creation  mask  is  set to the octal value nnn (see umask(2)).  If nnn is omitted, the current value of the mask is
	      printed.

       wait [n]
	      Wait for the specified process and report its termination status.  If n is not given,  all  currently  active  child  processes  are
	      waited for.  The return code from this command is that of the process waited for.

       Invocation.
       If  the	first  character  of  argument zero is -, commands are read from $HOME/.profile, if such a file exists.  Commands are then read as
       described below.  The following flags are interpreted by the shell when it is invoked.
       -c string  If the -c flag is present, commands are read from string.
       -s	  If the -s flag is present or if no arguments remain then commands are read from the standard input.  Shell output is written	to
		  file descriptor 2.
       -i	  If  the  -i  flag  is  present  or if the shell input and output are attached to a terminal (as told by gtty) then this shell is
		  interactive.	In this case the terminate signal SIGTERM (see sigvec(2)) is ignored (so that 'kill 0' does not kill  an  interac-
		  tive	shell)	and  the  interrupt  signal SIGINT is caught and ignored (so that wait is interruptible).  In all cases SIGQUIT is
		  ignored by the shell.

       The remaining flags and arguments are described under the set command.

FILES

       $HOME/.profile
       /tmp/sh*
       /dev/null

SEE ALSO

       csh(1), test(1), execve(2), environ(7)

DIAGNOSTICS

       Errors detected by the shell, such as syntax errors cause the shell to return a non zero exit status.  If  the  shell  is  being  used  non
       interactively  then  execution  of  the shell file is abandoned.  Otherwise, the shell returns the exit status of the last command executed
       (see also exit).

BUGS

       If << is used to provide standard input to an asynchronous process invoked by &, the shell gets mixed up about naming the  input  document.
       A garbage file /tmp/sh* is created, and the shell complains about not being able to find the file by another name.

7th Edition							    May 5, 1986 							     SH(1)